Washing machine

ABSTRACT

In a drum-type washing machine which is so designed that the speed of the drum will automatically be reduced to the initial low level if the amount of unbalance in the drum is excessively large at the time when the operation of the machine is to be shifted to a dehydration operation, a device is provided for accurately detecting the amount of unbalance in the drum is in terms of the amplitude of the vibration of the drum when said drum is driven at a speed intermediary between the washing speed and the dehydration speed.

United States Patent Katsumasa Matsuura Hitachi-shi; Hiroki Utsumi. Kashiwa-shi. both of, Japan Inventors Appl. No. 867,825 Filed Oct. 20, 1969 Patented June 8, 1971 Assignee Hitachi, Ltd.

Tokyo, Japan Priority Oct. 23, 1968 Japan 43/76775 WASHING MACHINE 7 Claims, 6 Drawing Figs.

11.5. C1 68/23.l, 210/144 Int. Cl D061 37/20 Field of Search 68/12, 24,

[ References Cited UNITED STATES PATENTS 2,760,639 8/1956 Haverstock 68/24X 2,911,812 11/1959 Metzger 68/12(R) 3,003,090 10/1961 Neyhouse et al. 68/12(R)X 3,033,014 5/1962 Neyhouse et al. 68/24X 3,226,960 1/1966 Mellinger 68/23.1

Primary Examiner-William I. Price Attorney-Craig, Antonelli, Stewart & Hill ABSTRACT: In a drum-type washing machine which is so designed that the speed of the drum will automatically be reduced to the initial low level if the amount of unbalance in the drum is excessively large at the time when the operation of the machine is to be shifted to a dehydration operation, a device is provided for accurately detecting the amount of unbalance in the drum is in terms of the amplitude of the vibration of the drum when said drum is driven at a speed intermediary between the washing speed and the dehydration speed.

PATENTED JUN 8|97| SHEET 1 0r 5 INVENTORS A'ATSuMAS/J MArsuurm an! HIROKI ul'sumt @(L m u/w ATTORN EYS PATENTEU JUN 8m: 3583182 SHEET 2 BF 5 INVENTORS KATSuMASA MHTSuuRA Nd HLHQKJ. ursuml PATENTEUJUN 81971 v 3583182 snmuoFs INVENTORS KRTSuMASA MATsuuxn 4nd MIND/{l HTJMML' ATTORNEYS WASHING MACHINE The present invention relates to a drum-type washing machine, and more particularly relates to such a drum-type washing machine which is so designed as to perform operations, such as a washing operation. a rinsing operation and a dehydration operation, automatically according to an established program, and which is provided with means for accurately detecting the amount of excessive unbalance occur ring in the drum at the outset of the dehydration operation.

A drum-type washing machine in which washing and dehydration of clothes are performed by rotating a drum about a horizontal or substantially horizontal shaft, has the advantage that the dehydration operation can be performed successively to the washing operation by increasing the rotational speed of the drum after the water has been drained. Therefore, such washing machine demonstrates an excellent performance as an automatic washing machine to perform a series of operations from a washing operation to a dehydration operation automatically according to an established program. In the washing machine of the type described, however, since the cleaning of the clothes, during the washing and rinsing operations, is achieved by the shock imposed on the clothes when the clothes gravitationally drop from a certain height to which they are hoisted by the rotating drum, the clothes are gathered in the bottom ofthe drum when the drum is to be accelerated from the washing operation to the dehydration operation and thus creates a large amount of unbalance with respect to the center of rotation ofthe drum.

Therefore, in the conventional washing machines of this type it has been practiced to detect the amplitude ofthe vibration of the drum in the process of acceleration at the initial stage of the dehydration operation and drop the rotational speed of the drum to the level for the washing operation, if the detected value of the amplitude is greater than a predetermined value, to change the distribution of the clothes within the drum, before the drum is accelerated again.

According to this method, the possibility of the drum being accelerated with a small amount of unbalance becomes great, because if the drum amplitude is greater than an allowable value in accelerating the drum for the first time, the rotational speed of the drum is dropped once to change the cloth distribution and the drum is again accelerated thereafter. Namely, even if the amplitude of the drum is greater than the allowable value in the process of acceleration, the transfer of the operation to the dehydration operation can be accomplished with a relatively small amount of unbalance while the acceleration and deceleration are repeated several times.

However, for practicing this method means must be provided for detecting the amplitude of the vibration of the drum or a water tub concentric with said drum, and in the past, use has been made of a microswitch which is actuated upon detecting an absolute displacement of the water tub. However, since the amplitude of the vibration becomes larger with the rotational speed of the drum, even when the amount of unbalance remains unchanged, if the amount of unbalance is close to the allowable value, the microswitch will be actuated only after the rotational speed of the drum has become considerably high. Therefore, a large stress occurs in the drum and the drum must be made from a thick plate of steel.

Further, in a common drum-type washing machine the moving parts which will cause a vibration of the drum and water tub, are elastically suspended from an outer frame and a vibration system is formed by the mass of'such moving parts and suspension springs. It is a common practice to select the critical speed of the vibration system to be lower than the rotational speed of the drum in the dehydration operation and to accelerate the drum abruptly from the washing speed to the dehydration speed so as to pass said point instantaneously. In the conventional drum-type washing machine, therefore, when the phase of vibration was displaced in the process of acceleration, the drum has occasionally been shifted to a high speed operation as a result of the microswitch being not actuated, in spite of the fact that a vibration large enough to actuate the microswitch has actually occurred. In order to avoid such drawback, it is considered to use a microswitch ofhigher sensitivity but this is not entirely satisfactory because, if the allowable value of amplitude is small, the microswitch will be actuated even in the presence of a small amount of unbalance when the drum is shifted to a high speed operation, and thus acceleration and deceleration will be repeated many times. Namely, the most serious drawback of the method of detecting an absolute displacement of the drum is that the microswitch is actuated even in a minute amount of unbalance and hence a lengthy time is required before the rotational speed of the drum is transferred to the dehydration speed, unless the allowable value of the amplitude of vibration is selected to be large.

An object of the present invention is to detect accurately the amount ofunbalance at a point where the rotational speed of the drum is relatively low.

Another object of the present invention is to prevent a malfunction ofa washing machine after said washing machine has been set in a dehydration operation, by detecting the vibration of a drum by a detector while said drum is rotating at a low speed and thereby precluding a malfunction of said detector after the drum has been shifted to a high speed rotation.

Still another object of the present invention is to provide a vibration detector at a low cost which has a long useful life.

The characteristic feature of the present invention resides in the fact that a drum is rotated for a predetermined period at a speed intermediary betweena rotational speed for a washing and rinsing operations and a rotational speed for a dehydration operation and the drum is accelerated to the rotational speed for the dehydration operation only when the amplitude of the vibration of the drum is below an allowable value during said period, said vibration detector being held against operation in the process and subsequent to the acceleration.

it is also an important feature of the present invention that a commercial mercury switch is attached obliquely on a water tub to be used as a vibration detector for detecting the vibration.

FIG. 1 is a front elevation ofa drum-type washing machine, with portions thereof broken away to show the interior mechanism;

FIG. 2 is a vertical cross-sectional side view of the drumtype washing machine;

FIG. 3 is a detailed cross-sectional view ofa vibration detector;

H0. 4 is a control circuit diagram of the washing machine;

FIG. 5 is a block diagram showing the operation of the washing machine; and

FIG. 6 is a graphic representation of the vibration characteristics of a moving portion consisting of a drum and a water tub.

With reference to the drawings, horizontal stays 2 are connected to the upper portions of the sidewalls, within a boxshaped outer frame 1. Each stay 2 is formed with hemispherical concaved surface 3 with a predetermined distance therebetween. In the hemispherical concaved surface 3 is mounted apiece 5 having a hemispherical convexed surface complementary to said hemispherical concaved surface, and a bolt 4 extends through said piece 5 and said hemispherical concave surface 3. The bolt 4 has a cylindrical spring seat 6, having a helical groove formed in the peripheral surface thereof, connected to the lower end thereof in threadable engagement therewith and a coil spring 7 is fitted in said helical groove at the top end thereof. The lower end of the coil spring 7 extends downward and is anchored to a fixture welded to the sidewall of a water tub 8. The water tub 8 is cylindrical in shape and suspended from the stays 2, fixed to the outer frame 1, by four of said coil springs 7.

Extending through a rear wall 10 of the water tub 8 is a bearing bush 9 and ball bearings ll are mounted in the oppos'ite ends of said bearing bush 9 to support a shaft 12. A

drum 14 is fixed on the inner end of the shaft 12, located interior ofthe water tub 8, by means ofa flange 13, whilst a pulley I6 is mounted on the outer end of said shaft l2 and fixed thereto by means of a key 15. On the inside wall of the drum 14 are fixed three radially inwardly projecting lifters 18 through the detailed structure thereof is not apparent in the drawings. In order to reinforce the drum 14, a stay bolt [9 is provided in the air space inside each of the lifters 18, extending axially of the drum. The drum 14 has a large number of water passage holes bored 17 through the peripheral wall thereof, through which water is drained in the dehydration operation. The water tub 8 has a front cover 20 connected water tight to the front side thereof by a packing 21. The front cover 20 has an opening at the center thereof for throwing clothes into the water tub 8 therethrough, and a bellows 22 is provided between said opening and an opening 23, formed in the outer frame I in registration with said first opening, with the opposite ends thereof connected to said respective openings, so as to prevent water from dripping between the outer frame 1 and the front cover 20 when the clothes are charged into or removed from the water tub. The bellows 22 is made of a relatively soft material so that the vibration of the water tub 8 may not be transmitted to the outer frame 1. The opening 23 in the outer frame 1 is closed with a transparent door 25. Arms 26 and 27 extend downward from the underside ofthe water tub 8 to mount a dehydration motor 29 and a washing motor 28 thereon respectively. A large pulley 32 is loosely mounted on a portion of and a pulley 33 is fixedly mounted on the outer end ofthe drive shaft 30 ofthe dehydration motor 29. The large pulley 32 is operatively connected to the pulley 33 through a coil spring clutch 31 and further to the washing motor through a V-belt. The pulley 33 and a pulley 16 mounted on the shaft 12 are operatively connected with each other by a V-belt 34. The washing motor is a speed variable motor ofe.g. pole change type and drives the drum [4 through the large pulley 32, the pulley 33, the V-belt 34 and the pulley l6 and also drives a drain pump 35 through a V-belt 36, said drain pump being mounted below the water tub 8. Friction plates 37, 38 are fixed to the water tub 8 providing outwardly therefrom and each of which is clamped by sliding members 40, 41 from both sides thereof which are fixed to the top end of brackets 39. Thus, it will be seen that a vibration of the water tub 8 in the same plane as the friction plates 37, 38 are damped by the friction between said friction plates and the sliding members 40, 41.

A water feed flange 42 is mounted on the upper portion of the sidewall of the outer frame 1 and an electromagnetic valve 43 is connected to said flange 42. The downstream end of the electromagnetic valve 43 is communicated with the upper portion of the water tub 8 through a hose 44. The level of the water fed in the water tub 8 is detected by a pressure switch 45 through a conduit 46 which is connected to the lower portion of said water tub 8. On the top wall ofthe outer frame 1 is provided a manually operative knob 48 and a timer 47 is provided coaxially with said knob to be Operated thereby. The timer 47 accommodates therein a timer motor to be described later and this timer motor is automatically set in operation when the knob 48 is set on a start position.

A vibration detector 49 is mounted on the upper portion of the front wall of the water tub 8 and enclosed by a cover 50 so that a mercury switch may be protected against breakage caused by an external shock. FIG. 3 shows the detailed structure of the vibration detector. As shown, the vibration detector consists of a commercial mercury switch having mercury 52 and a pair of electrodes 53 sealed in a glass container 51, and mounted in a tubular case 55 with a buffer 54, such as sponge or rubber, interposed therebetween. A lead wire 56 connected to each electrode 53 is led to the outside through a hole 57 formed in the case 55. In order that the lead wire 56 may not be disconnected from the electrode even when subjected to an external force, the connecting portion thereof is folded and the folded end is covered with an insulating sleeve 58, such as a vinyl pipe, and further the outer end of said insulating sleeve is secured by a cap 59 screw-threaded into the open end of the case 55, whereby said respective parts are securely held within the case 55. The case 55 has an integral base 60 and is fixed on the front end wall of the water tub 8 at said base by screws 61 at a predetermined angle 0 with respect to a horizontal line, as shown in H05. 2 and 3. The angle 0 of inclination is selected such that the natural frequency of a vibration system, formed by the mercury 52 and the container 51, will substantially coincide with the primary critical speed ofa moving part which will be described hereinafter.

In a washing machine ofthe type in which the water tub 8 is elastically suspended by the coil springs 7, a vibration will occur during operation which is composed of vibrations in a horizontal direction and in a vertical direction as viewed in FIG. 1 and in a direction normal to the sheet of FIG. I, that is, the direction ofthe rotary shaft of the drum, and vibrations to cause the drum to rotate about said respective directions, all of which are complicatedly combined with each other. Of these vibrations the one which is largest in amplitude is the vibration in a horizontal direction, namely the vibration to cause the water tub 8 to swing horizontally in the suspended state by the coil spring 7. In general, the natural frequency or the critical speed at this time is so selected as to be greater than the rpm. during the washing operation but smaller than the rpm. during the washing operation but smaller than the rpm. during the dehydration operation. This is because of minimizing the vibration of a moving part, such as the water tub 8, being transmitted to the outer frame and of making the amplitude of the moving part as small as possible during the washing and dehydration operations. The primary critical speed is generally determined by the mass of the moving part and the length of the coil springs, and is from 100 to I50 r.p.m. Therefore, the mounting angle of the vibration detector switch is so selected that the natural frequency of the vibration system may coincide with the upper limit of said critical speed. The natural frequency of the vibration detector as shown in H0. 3 will not become a completely independent vibration system but the vibration can be detected with such an accuracy as will not substantially endanger the operation of the machine, by selecting the angle 0 to be 15 relative to the critical speed.

Now, the control circuit of the present washing machine, shown in FIG. 4 will be explained. A door switch 63 and a cam switch 64 to be controlled by a cam are connected in series to a power source 62. Cam switches 64, 65, 66, 67 and 68 are controlled by a series of cam plates to be driven by a timer motor 69 and the circuits thereof are closed for periods indicated by the thick horizontal lines shown in FIG. 5. The cam switch 68 is a reversing contact to operate the washing motor 28 in opposite directions alternately in a short period of about 30 seconds. A relay 70 is connected to a contact a of the cam switch 65 to control relay contacts 71, 72. A normally closed contact NC ofthe pressure switch 45 is connected to the electromagnetic valve 43 for feeding water, whereas a normally open contact NO thereof is connected to the timer motor 69. A relay 74 is provided in parallel to the dehydration motor 29 to control contacts 75, 76 thereof. An auxiliary timer 77, connected in series with the cam switch 67, control timer contacts 84, 85. These timer contacts repeat a cycle to close the circuit only for a short period of 2-3 seconds throughout the period when the auxiliary timer 77 is in operation, with a certain time delay, e.g. a time delay of l0 seconds. To a contact b of the cam switch 66 is connected a drain valve 78 and a relay 79 to control relay contacts 80, 81 and 82. The relay contact 82 is to change the pole number of the washing motor 28, and is connected such that a large pole number will be obtained on the NC side and a small pole number on the NO side. The vibration detector 49 is connected in series to the relay contact 80. A pilot lamp 83 is connected in series to the door switch 63 and the cam switch 64, and indicate the washing machine being in operation.

washing agent and after closing the door 25, the knob 48 is set on the start position. The electromagnetic valve 43 is cnergized to feed water into the drum, by a current supplied thereto from the NO side of the relay contact 72 through the NC side of the pressure switch 45. When the surface of the water in the water tub has reached to a prescribed level, the pressure switch 45 is turned to the NO side, thereby the timer motor 69 is energized and the washing motor is set in operation through the NO side of the relay contact 71. Since the relay contact 82 is on the NC side at this time, the motor 28 rotates on the multipole side, namely at a low speed.

The r.p.m. of the drum 14 at this time is 50. After the washing operation for a predetermined period the machine is shifted to a draining operation and the relay 70 is deenergized, so that the relay contact 72 engages the NC side and the drain valve 74 is operated to drain the washing water. In this case, the relay 79 is energized but no problem will rise even when the contact 82 is switched, because the washing motor 28 is disconnected from the power source. The rinsing operation which will follow the draining operation takes place in exactly the same manner as described above. When the draining operation has set in and the water in the water tub 8 has been drained, the cam switch 66 is shifted from the contact b to a and the cam switch 67 closes the circuit, so that the auxiliary timer 77 is actuated. At the same time, the washing motor 28 is supplied with a current through the NC side of the relay contact 75 and rotates in one direction at a low speed. By the actuation of the auxiliary timer 77, the timer contact 84 closes the circuit for a short period oftime, whereby the relay 79 and the drain valve 78 are energized. Upon energization of the relay 79, the contact 82 is switched to the NO side and thus the washing motor 28 is shifted to the small pole number side and accelerates the drum 14 to 100 r.p.m. The timer contact 84 closes the circuit only for 2-3 seconds. Therefore, if a large vibration occurs in the process of acceleration, the vibration detector 49 is opened to release the relay 79 and the drain valve 78 from the energized state. Thus, the washing motor 28 is placed on the small pole number side and rotates the drum 14 at a low speed once again. During the low speed operation of the drum, the clothes are free to tumble within said drum and, therefore, the distribution of the clothes is changed before the timer contact 84 is closed again. Upon lapse of a certain time, the timer contact 84 is closed again closes the circuit for a short period of time and the washing motor 28 is shifted to a high rotation. If the vibration of the drum or the water tub is below an allowable value or, in other words, the vibration is not so intense as will cause the circuit of the vibration detector 49 to be opened, even after the washing motor has been shifted to the high speed operation, the relay 79 will be continuously energized to maintain the contacts 80, 81 and 82 on the NO side even when the timer contact 84 is opened. In a predetermined time after the timer contact 84 has been opened, the timer contact 85 is closed to energize the dehydration motor 29 and the relay 74. The energization of the relay 74 causes the contacts 75, 76 to engage the N0 side, so that the washing motor 28 is disconnected from the power source and the drum 14 is driven from the dehydration motor 29 at a high speed, e.g. at 700 r.p.m. The drain valve 78 and the relay 79 are also energized through the relay contact 76 and the drum will not be returned to the low speed operation even if the circuit of the vibration detector 49 is opened in a half way. Namely, the present washing is so designed that once the dehydration motor 29 has been actuated, the machine performs the dehydration operation for a predetermined period even if the vibration detector 49 is actuated in a half way.

Here, the vibration detector 49 actually detects the vibration of the water tub 8 only for a very limited period from the time when the relay 79 is actuated to the time when the timer contact 85 is closed, and if the vibration of the water tub is below the allowable value within this period, it is assured that the vibration be maintained within the allowable range throughout the subsequent operation. This is because of the following reason: Namely, when the drum 14 has been accelerated to an intermediate speed upon actuation ofthe relay 79, it is operated for a predetermined period at a speed approximating to the critical speed ofthe vibration system ofthe washing machine. Therefore, even if the amount of unbalance within the drum is relatively small, the vibration will be detected by the vibration detector as a large amplitude. Furthermore, at the intermediate speed the clothes are attached to the inside wall of the drum under the influence of centrifugal force and hence the distribution of the clothes will not undergo any change even when the rotational speed of the drum is further elevated.

in the vibration characteristic curves shown in FIG. 6 of the washing machine, the vibration characteristic curve in a horizontal direction has the critical speed fnh at about I00 r.p.m. and the vertical characteristic curve 91 in a vertical direction has the critical speed fnv at about 200 r.p.m. Under such condition, the intermediate speed is preferably selected in the range from a rotational speed at which the clothes are substantially attached to the inside wall of the drum, to a speed which is intermediary between the critical speed in a horizontal direction and that in a vertical direction. The lower limitf of the intermediate speed is variable depending upon the drum diameter but is about 75 r.p.m. for a drum having a diameter of 450 mm. and the upper limitf thereof is about r.p.m. The upper limit f, of the rotational speed is preferably selected between the primary critical speed fnh and the secondary critical speed fmv, though variable depending upon the spring constant of the coil springs 7 and the mass of the moving part. When the intermediate rotational speed is selected in the range specified above, it is desirable that the natural frequency of the vibration detector is higher than the intermediate rotational speed. Namely, the vibration detector 49 is to be used as an acceleration-type vibrometer. Therefore, by rotating the drum for a certain period at the intermediate speed, the vibration of the drum can reliably detected even if a phase deviation occurs. if the amplitude of the water tub is smaller than the allowable value at the intermediate speed, the amplitude of the vibrating water tub at the final dehydration speedf (FIG. 6) will be small in both a horizontal direction and a vertical direction. Since the vibration is accelerated in proportion to the rotational speed, it may be considered that the circuit of the vibration detector 49 would be opened at the final dehydration speed. In practice, however, the vibration detector 49 is shorted, once the drum has been set in the high speed operation, and hence becomes substantially inoperative, whereby an erroneous operation is prevented. The acceleration to the intermediate speed is effected by an instruction from the auxiliary timer 77 and it may be considered that the drum would be accelerated when the cloth distribution is very nonuniform. However, since the intermediate speed is much lower than the dehydration speed, the external force acting on the drum 14, etc. becomes small. ln the practical operation, the vibration detector 49 is opened in the process of acceleration when the amount of unbalance is large and the speed of the drum is reverted to the original washing speed at a point where the r.p.m. of the drum is lower than the r.p.m. at the intermediate speed.

It may also be considered that when uniform cloth distribution cannot be attained for a prolonged time due to the type of the clothes charged in the drum, the speed of the drum would reciprocate between the intermediate speed and the washing speed, and the final dehydration speed would not be obtained. However, the experiment conducted on the actual washing machine has proved that such condition would not occur unless the machine is intentially operated under specific conditions.

Upon completion of the dehydration operation, the rinsing and draining operations are repeated several times before the final dehydration operation.

After the final dehydration operation, the drum is rotated at a speed same as the washing speed to relax the clothes which have strongly been pressed against the inside wall of the drum in the dehydration step, and thereby to remove the creases set in the clothes. and by this operation the entire washing operation is accomplished.

As shown in FIG. 3, the vibration detector 49 can be incorporated in the washing machine, simply by accommodating a commercial mercury switch in the case 55 and obliquely mounting the case on the water tub. It is not necessary to use a special vibration detector. Therefore. the vibration detector can be provided at a low cost and yet is capable ofinterrupting a current sufficient to operate the drain valve since the current capacity of the mercury switch can be made relatively large.

What we claim is:

l. A washing machine comprising a drum elastically supported relative to an outer frame and adapted for accommodating clothes, means for driving said drum at least at three different speeds stepwise, means for detecting the vibration of a moving part only when the drum is driven at the intermediate speed and means for accelerating the drum to the higher speed and rendering said vibration detecting means substantially inoperative when the vibration detected by said vibration detecting means remains below an allowable values for a predetermined period.

2. A washing machine as defined in claim I. wherein said in termediate speed is selected to be higher than a speed at which the clothes are attached to the drum wall under the influence of centrifugal force but lower than the secondary critical speed of a vibration system composed by the moving part and an elastic member supporting said moving part.

3. A washing machine as defined in claim 1, wherein said vibration detecting means consists of a conventional mercury switch mounted on the moving part at an angle to a horizontal line, the electrodes of said mercury switch being opened electrically by the vibration ofthe moving part when said vibration is larger than the allowable value.

4. A washing machine as defined in claim 3, wherein the natural frequency of said mercury switch is either the same as or slightly greater than the frequency ofthe drum driven at the intermediate speed.

5. A washing machine comprising a drum elastically supported relative to an outer frame and adapted for accommodating clothes. means for driving said drum at least at three different speeds stepwise, means for detecting the vibration of a moving part only at the initial stage ofa dehydration operation and auxiliary timer means for setting a dehydration motor in operation when said vibration detecting means has not detected a vibration larger than an allowable value for a predetermined time after said drum had been accelerated to the intermediate speed.

6. A washing machine as defined in claim 5, wherein said auxiliary timer means is driven by an auxiliary timer motor and has two contacts to be closed alternately in an predetermined time interval, and starts the dehydration motor when the vibration detected by the vibration detecting means in a period from the time when one of said contacts was closed to the time when another one of said contacts is closed is below the allowable value.

7. A washing machine comprising a drum for accommodating clothes, at least two motors for driving said drum, speed changing means for driving the drum at an intermediate speed higher than a washing speed but lower than a dehydration speed, auxiliary timer means for changing the drum speed from the washing speed to the intermediate speed and means for reverting the drum speed to the initial washing speed when the vibration of a moving part at the intermediate speed is larger than an allowable value, a dehydration motor being set in operation and the circuit of vibration detecting means being shorted by the contact of auxiliary timer means when a condition of said vibration being below the allowable value has lasted for a predetermined time and the contact of said auxiliary timer means has beenclosed. 

1. A washing machine comprising a drum elastically supported relative to an outer frame and adapted for accommodating clothes, means for driving said drum at least at three different speeds stepwise, means for detecting the vibration of a moving part only when the drum is driven at the intermediate speed and means for Accelerating the drum to the higher speed and rendering said vibration detecting means substantially inoperative when the vibration detected by said vibration detecting means remains below an allowable values for a predetermined period.
 2. A washing machine as defined in claim 1, wherein said intermediate speed is selected to be higher than a speed at which the clothes are attached to the drum wall under the influence of centrifugal force but lower than the secondary critical speed of a vibration system composed by the moving part and an elastic member supporting said moving part.
 3. A washing machine as defined in claim 1, wherein said vibration detecting means consists of a conventional mercury switch mounted on the moving part at an angle to a horizontal line, the electrodes of said mercury switch being opened electrically by the vibration of the moving part when said vibration is larger than the allowable value.
 4. A washing machine as defined in claim 3, wherein the natural frequency of said mercury switch is either the same as or slightly greater than the frequency of the drum driven at the intermediate speed.
 5. A washing machine comprising a drum elastically supported relative to an outer frame and adapted for accommodating clothes, means for driving said drum at least at three different speeds stepwise, means for detecting the vibration of a moving part only at the initial stage of a dehydration operation and auxiliary timer means for setting a dehydration motor in operation when said vibration detecting means has not detected a vibration larger than an allowable value for a predetermined time after said drum had been accelerated to the intermediate speed.
 6. A washing machine as defined in claim 5, wherein said auxiliary timer means is driven by an auxiliary timer motor and has two contacts to be closed alternately in an predetermined time interval, and starts the dehydration motor when the vibration detected by the vibration detecting means in a period from the time when one of said contacts was closed to the time when another one of said contacts is closed is below the allowable value.
 7. A washing machine comprising a drum for accommodating clothes, at least two motors for driving said drum, speed changing means for driving the drum at an intermediate speed higher than a washing speed but lower than a dehydration speed, auxiliary timer means for changing the drum speed from the washing speed to the intermediate speed and means for reverting the drum speed to the initial washing speed when the vibration of a moving part at the intermediate speed is larger than an allowable value, a dehydration motor being set in operation and the circuit of vibration detecting means being shorted by the contact of auxiliary timer means when a condition of said vibration being below the allowable value has lasted for a predetermined time and the contact of said auxiliary timer means has been closed. 